1. Field of the Invention
The present invention relates to the production of supported catalysts, more specifically to the method for making heterogeneous catalysts containing nano-meter sized metal particles.
2. Description of the Related Art
Many industrial products such as fuels, lubricants, polymers, fibers, drugs, and other chemicals would not be manufacturable without the use of catalysts. Catalysts are also essential for the reduction of pollutants, particularly air pollutants created during the production of energy and by automobiles. The majority of industrial catalysts are composed of a high surface area support material upon which chemically active metal nanoparticles are dispersed. The support materials are generally inert, ceramic type materials having surface areas on the order of hundreds of square meters/gram. This high specific surface area usually requires a complex internal pore system. The metal nano-particles are deposited on the support and dispersed throughout this internal pore system, and are generally between 1 and 100 nanometers in size. Catalysts of this type are also referred to as heterogeneous catalysts, because the catalyst particles are solid phase, while the reactants interacting with the catalyst are generally liquid or gas phase.
Processes for making supported catalysts go back many years. One such process for making platinum catalysts, for example, involves the contacting of a support material such as alumina with a metal salt solution such as a platinum chloride solution. The salt solution “impregnates” or fills the pores of the support during this process. Following the impregnation, the support containing the salt solution would be air dried, causing the metal salt to precipitate within the pores. The support containing the crystallized metal salt would then be exposed to a hydrogen or carbon monoxide gas environment, reducing the solid metal salt to metal particles. This process, however, made it difficult to produce highly dispersed catalysts because of the difficulty in controlling the precipitated metal salt crystallite sizes and distributions. Often, depending on the reduction conditions, the metal particles would diffuse together creating larger, less desirable particles (sintering).
With the advent of the more recent focus of nanotechnology, methods for fabricating nanometer sized metal particles in liquid solutions have been combined with impregnation techniques to create heterogeneous catalysts. This process offers the potential advantage of being able to determine metal particle size, morphology and particle size distribution prior to impregnation into the support.
Yoo et al., in an article entitled “Propene Hydrogenation Over Truncated Octahedral Pt Nanoparticles Supported on Alumina”, Journal of Catalysis, 214 (2003), pg 1-7, discloses a process for loading colloidal Pt nanoparticles (synthesized by a 1:5 concentration ratio of K2PtCl4 to polyacrylate capping polymer) into an alumina support via impregnation.
Miyazaki et al., in an article entitled “Morphology Control of Platinum Nanoparticles and Their Catalytic Properties”, Journal of Nanoparticle Research, Vol. 5, pg 69-80, 2003, discloses the preparation of Pt nanoparticles of varying morphology through the use of different capping polymers. Various shapes (such as square, triangular, and hexagonal) of platinum crystallites, as observed by transmission electron microscopy, were obtained. Supported catalysts were made by impregnation of previously formed Pt crystallites into an alumina support. Water was removed from the support by freeze drying, and the capping polymers were removed by calcining in air at 500° C. for 8 hours.
U.S. Pat. No. 6,569,358 discloses a method of preparing a porous material incorporating ultrafine metal particles comprising the following steps: (1) preparing surface-protected ultrafine metal particles by reducing metal ions in the presence of molecules such as dodecanethiol molecules; (2) immersing a wet gel in a solution of the ultrafine metal particles, thus forming an ultrafine metal particle/wet gel composite in which the ultrafine metal particles are incorporated in the wet gel; and (3) drying the ultrafine metal particle/wet gel composite to form a porous body.
The aforementioned processes utilize a protecting agent, or capping polymer, to control particle size, morphology, and reduce agglomeration. However, removal of the capping polymers or protecting agents can be an issue for sensitive catalytic processes, as their destruction may leave contaminating residues that are undesirable. These residues may reduce activity of the catalyst by occupying active sites necessary for subsequent reactions. The residues may also leave behind trace quantities of poisons that will eventually kill the catalyst over time. Removal of organic capping agents and polymers usually require oxidation (or burning), the exothermic heat from which can produce unwanted sintering due to the high temperatures. Sintering will increase the metal particle size and reduce the active surface area which is undesirable. Furthermore, the use of capping agents can hinder the introduction of the metal crystallites into small pores of the support.
U.S. Pat. No. 6,686,308 discloses a supported catalyst comprising catalyst metal nanoparticles having an average particle size of typically 2.0 nm or less, which are supported on support particles at a loading of 30% or more. Typical catalyst metals are selected from platinum, palladium, ruthenium, rhodium, iridium, osmium, molybdenum, tungsten, iron, nickel and tin. Typical support particles are carbon. A method of making a supported catalyst comprises the steps of: a) providing a solution of metal chlorides of one or more catalyst metals in solvent system containing at least one polyalcohol, typically ethylene glycol containing less than 2% water; b) forming a colloidal suspension of unprotected catalyst metal nanoparticles by raising the pH of the solution, typically to a pH of 10 or higher, and heating said solution, typically to 125° C. or higher; c) adding support particles to the colloidal suspension; and d) depositing the unprotected catalyst metal nanoparticles on the support particles by lowering the pH of said suspension, typically to a pH of 6.5 or lower.
U.S. Pat. No. 6,603,038 discloses a method for producing a catalyst containing one or several metals from the group of metals comprising the sub-groups Ib and VIIIb of the periodic table on porous support particles, characterized by a first step in which one or several precursors from the group of compounds of metals from sub-groups Ib and VIIIb of the periodic table is or are applied to a porous support, and a second step in which the porous, preferably nanoporous support to which at least one precursor has been applied is treated with at least one reduction agent, to obtain the metal nanoparticles produced in situ in the pores of the support. Catalysts were typically prepared by impregnation of the support with a metal salt solution, followed by a drying step. Subsequent to drying, the impregnated support materials were reduced by various techniques including re-impregnation with liquid reducing agents. Typically, the initial salt impregnation process was performed with support to salt solution ratios on the order of 1 g support/1 ml solution. These impregnation conditions are typical of traditional prior art, and generally result in lower dispersions and poor control of particle sizes and particle size distributions.
What is needed is a catalyst manufacturing process that provides improved control over metal crystallite particle sizes, distributions and morphologies without the contamination of capping agents.